Here is what I found on AskUbuntu:$ sudo cp /usr/share/applications/gnome-appearance-properties.desktop /usr/share/gdm/autostart/LoginWindow
Log out, you will be prompted with the Appearance window. Change the theme as you change it for the desktop. Log in.$ sudo rm /usr/share/gdm/autostart/LoginWindow/gnome-appearance-properties.desktop
Please, report here if it works.
